Don't call them love rats. Virtuous voles turn out to be all too human

They have been the pin-up boys of the moral right for decades: prairie vole males look after babies, build nests, stick to one partner - and make ideal models for humans. It was even thought that studies of their brain chemistry, which indicated that their monogamy appeared to be controlled by hormones, suggested ways in which drugs could help to cure errant humans.

But this convenient notion has been demolished by scientists who discovered that 'monogamous' prairie voles are really just a bunch of randy rodents. A study published in Animal Behaviour found the prairie vole, Microtus ochrogaster, displayed considerable sexual promiscuity. Almost a quarter of litters were found not to have been fathered by the live-in partner of the mother prairie vole.

'There is a difference between social monogamy and sexual fidelity,' said the study's main researcher, Professor Alex Ophir, of Florida University, Gainesville. 'You can pair with a partner for life and still have sex with others - and that is what prairie voles do. There is a lesson there for humans.'

The discovery of prairie vole promiscuity is crucial because these animals are favourite subjects among researchers, selected because they had displayed life-long monogamy. These previous studies also showed that dopamine, a brain chemical released during sex, played a key role in determining vole sexual behaviour.

Dopamine - the vole's love drug - causes males to lose interest in other females and acts on the nucleus accumbens, a region in the forebrain of many animals, including humans. Previous studies claimed dopamine locked the vole into monogamy and, by inference, played a similar role in humans - an idea that promised new ways of understanding, and possibly treating, serial promiscuity.

According to Nature last week, Eric Keroack, then head of a US government family planning committee, used the monogamous prairie vole as evidence that people who had extramarital sex damaged their brain chemistry. But now the prairie vole has spoiled the whole idea by revealing its true nature - as a love rat.

Nor is the prairie vole the first creature presented as a pillar of society to have let down its supporters. The British dunnock, or hedge sparrow, was once considered a paragon of decency. A Victorian vicar even commended its faithfulness and for being 'sober and unpretending in its dress, while still neat and graceful, and so exhibits a pattern which many of a higher grade might imitate'.

The Cambridge zoologist Nick Davies, however, carried out investigations into dunnock society and found it riddled with sexual excess and deception. Most female dunnocks had 'extramarital' partners and sexual communes were common. 'Had the vicar's congregation followed his suggestion, there would have been chaos in the parish,' says Davies. It is a point backed by Ophir. 'If you draw parallels with animal society and human society, you do so at your peril,' he said.